1. Field of the Invention
This invention relates to a process for recovery of zinc products including essentially pure metal oxides and metals. The present invention more specifically relates to an improved method for the recovery of essentially pure zinc oxide from EAF dust.
2. Description of Related Art
Zinc oxide typically is a fine white or grayish powder which has a variety of uses including as a rubber accelerator, as a pigment, as a dietary supplement and in the semiconductor field. Zinc oxide is found in commercial by-products including waste material streams such as fly ash and flue dust. Methods for recovering zinc oxides are known in the art, including recovering zinc oxide from industrial waste materials. Such previous methods have included leaching with mineral acid, caustic soda, ammonium hydroxide, and ammonium carbonate solutions. However, these methods have low yields of zinc oxide and typically do not recover pure zinc oxide, the recovered zinc oxide being contaminated with other metal salts. Therefore, in order to obtain pure zinc oxide, subsequent reduction and washing processes were necessary.
U.S. Pat. No. 3,849,121 to Burrows discloses a method for the selective recovery of zinc oxide from industrial waste. The Burrows method comprises leaching a waste material with an ammonium chloride solution at elevated temperatures, separating iron from solution, treating the solution with zinc metal and cooling the solution to precipitate zinc oxide. The Burrows patent discloses a method to take EAF dust which is mainly a mixture of iron and zinc oxides and, in a series of steps, to separate out the iron oxides and waste metals. However, the material obtained in the last step is a mixture of a small amount of zinc oxide, hydrated zinc phases which can include hydrates of zinc oxide and zinc hydroxide, as well as other phases and a large amount of diamino zinc dichloride Zn(NH.sub.3).sub.2 Cl.sub.2 or other similar compounds containing zinc and chlorine ions. Currently, the Burrows method is not economically viable because of Environmental Protection Agency guidelines established subsequent to the issuance of the Burrows patent. Additionally, the Burrows method is not a continuous method and, therefore, is not economical as a continuous process.
U.S. Pat. No. 4,071,357 to Peters discloses a method for recovering metal values which includes a steam distillation step and a calcining step to precipitate zinc carbonate and to convert the zinc carbonate to zinc oxide, respectively. Peters further discloses the use of a solution containing approximately equal amounts of ammonia and carbon dioxide to leach the flue dust at room temperature, resulting in the extraction of only about half of the zinc in the dust, almost 7% of the iron, less than 5% of the lead, and less than half of the cadmium.
Steam distillation is contrary to dilution. Steam distillation precipitates zinc carbonate, other carbonates and iron impurities. Steam distillation also disadvantageously results in an increase in temperature which drives off ammonia and carbon dioxide, resulting in the precipitation of iron impurities and then zinc carbonate and other dissolved metals. The purity of the zinc carbonate obtained depends on the rate of steam distillation and the efficiency of solids separation as a function of time. Calcining converts the zinc carbonate to zinc oxide, whereas washing and drying at temperatures between 100.degree. C. and 200.degree. C. converts the zinc compounds to zinc oxide.
U.S. Pat. No. 5,464,596 to Myerson, commonly assigned with the present application, discloses a method for the recovery of zinc oxide by treating a waste stream with a 23% ammonium chloride at 90.degree. C., separating undissolved components from the solution, displacing undesired metal ions from the solution using zinc metal, cooling the solution to precipitated out zinc compounds, washing the precipitate to remove various soluble zinc compounds, leaving zinc oxide of greater than 99%. The '596 patent teaches that ammonium chloride solutions must be at least 90.degree. C. to sufficiently dissolve the zinc compounds. Heating an aqueous solution to such a temperature requires the expenditure of large amounts of energy. It is further taught that while NH.sub.4 Cl concentrations below 23% do not dissolve the maximum amount of zinc oxide from the waste material, concentrations greater than 23% result in an impure zinc oxide due to the tendency of the NH.sub.4 Cl to precipitate out of solution with the zinc compounds at such high concentrations. Furthermore, the cooling of the product solution results in the precipitation of various zinc species, resulting in crystallization of some species. Because of this, using the cooling step disclosed in Myerson '596, one cannot use concentrations of ammonium chloride solutions above about 23%, limiting the usefulness of the process disclosed in Myerson '596. Further, contaminates of the zinc oxide must be removed by an additional washing step.
U.S. Pat. No. 5,759,503 to Myerson, et al., commonly assigned with the present application, discloses a method for the recovery of zinc oxide by dissolving zinc oxide in an intermediate, diluting the intermediate by a factor of 3 to 30 by adding 70-100.degree. C. water, and filtering out the resultant zinc oxide crystals. The '503 patent, along with its family of patents, disclose using ammonium chloride solutions of 23% and teach that using higher concentration ammonium chloride solutions will produce undesired results. This was the understanding at the time of invention of the processes disclosed and claimed in the Myerson '503 patent and its family of patents. These undesired results include the precipitation of various zinc species, resulting in crystallization of some species. Because of this, using the cooling step disclosed in Myerson '596, one cannot use concentrations of ammonium chloride solutions above about 23%, limiting the usefulness of the process disclosed in Myerson '503 Although the processes disclosed and claimed in Myerson '503 patent are valuable and perform admirably, it has now been discovered that by altering the process steps, including the addition of a dilution step, ammonium chloride solutions of 30% or greater can be used to recycle industrial waste streams.
Therefore, there exists a need for an alternative method that will recover essentially pure zinc oxide from industrial waste materials that is economical, quick, and efficient and, optionally also will allow the recovery of elemental lead, cadmium, and copper from industrial waste materials, at the lower end of the temperature range and at higher leach solution concentrations than previously thought possible.